We have previously shown that defective herpes simplex virus (HSV) genomes which accumulated in serially passaged virus stocks consisted of multiple head-to-tail reiterations of limited subsets of the standard virus DNA sequences. Concatemeric packaged defective genomes could be derived by cotransfection of cells with helper virus DNA and cloned monomeric seeds (amplicons) containing a replication origin and a cleavage/packaging signal. HSV amplicons could also be used as vectors for the introduction of foreign DNA sequences into defective virus genomes, which were stably propagated in serially passaged virus stocks. Using tests for amplicon propagation we have mapped three replication origins in standard virus DNA: Two identical origins (ori-1 and ori-1') were found to map within the inverted repeat of the S component. The third replication origin (ori-2) mapped within the unique sequences of the L component. DNA sequences of ori-2 are deleted during bacterial cloning but are efficiently repaired during the cotransfection with helper virus DNA. We propose to continue our studies of ori-2 and to investigate the mechanism of the deletion-repair. These and other proposed studies should contribute to the understanding of the mechanism of HSV DNA replication. Defective genomes present in a series propagated by undiluted passaging of HSV-1 strain Justin were found to contain large repeat units, which included the Us viral DNA sequences encoding the glycoproteins D (gD) and E (gE). Cells infected with the defective virus stocks were found to overproduce the precursor to gE. However, no increased gD synthesis was noted. Analyses of defective genomes constructed to contain multiple gD gene templates have similarly revealed the regulation of the amount of gD made during viral infections. We now propose to further investigate the regulation of this important viral glycoprotein which plays a major role in the induction of the host immune response.